Make the batter: In a medium bowl, whisk together the egg, Greek yogurt, cinnamon, and vanilla extract. Stir in the protein powder and milk until smooth.
Dip the bread: Coat each slice of bread fully in the batter, making sure both sides soak it up.
Cook: Heat a greased skillet over medium heat. Cook bread slices for 2–3 minutes per side, until golden brown.
Assemble & enjoy: Stack slices, top with extra Greek yogurt, banana slices, and a drizzle of maple syrup if desired. Serve warm.